chore(ai): Add metric for tracking ai cost calculation

[shellmayr]

• Add a new metric in relay-event-normalization genai.cost_calculation.result to track positive, negative and zero costs using a metric
• Add finite tag matchers for the origin integration and platform, to prevent our cardinality from becoming too large
• In the cost calculation function normalize_ai_costs, emit that metric depending on the outcome

Closes TET-1675

[linear]

TET-1675 Add a metric in Relay for cost calculation

[Dav1dde]

Nice, just a tiny nit and what we discussed that it'll only cover a single pipeline not transaction and span streaming.

[Dav1dde]

As discussed in Slack, the change here would be only for the span streaming pipeline, not the transaction one (the only one in active use atm)

Maybe you can move the metric emission into ai::calculate_costs, then it covers both pipelines at once.

[Dav1dde]

This is a bit awkward because we already have such a function in relay-server, but no good way of sharing it :(

Fine to keep here, would consider just starting utils.rs module in the crate to hide it away, but also not a big deal, just would move it at the end of the file, generally we like to keep public interfaces at the top.

[jjbayer]

I would move this to statsd.rs module because it is a concern of metrics reporting (same for map_origin_to_integration).

[jjbayer]

Something like this might be more concise (untested):

Suggested change

	let metric_label = if (costs.input > 0.0 && costs.output > 0.0)
	|| ((costs.input > 0.0 && costs.output == 0.0)
	|| (costs.input == 0.0 && costs.output > 0.0))
	{
	"calculation_positive"
	} else if costs.input < 0.0 || costs.output < 0.0 {
	"calculation_negative"
	} else {
	"calculation_zero"
	};
	let metric_label = match (costs.input.signum(), costs.output.signum()) {
	(-1, _) | (_, -1) => "calculation_negative",
	(0, 0) => "calculation_zero",
	_ => "calculation_positive",
	};

[jjbayer]

I would move this to statsd.rs module because it is a concern of metrics reporting (same for map_origin_to_integration).

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

^{Bugbot Autofix is OFF. To automatically fix reported issues with Cloud Agents, enable Autofix in the Cursor dashboard.}

[Dav1dde]

I assume removing that comment was a mistake?

[shellmayr]

yup, adding it back in